The invention starts with a protective circuit for the protection of a subscriber line circuit against overvoltage, as described in the European patent application EP 0242562 B1.
Subscriber line circuits generally need protective circuits to protect components or modules that are sensitive to overvoltage against excess voltage. Such overvoltage is often caused by atmospheric discharges or by the magnetic coupling of power lines, and is pulse-shaped or sinusoidally-shaped in nature.
The European patent application EP 0242562 B1 contains a protective circuit in the subscriber line before the part of the subscriber line circuit to be protected, which comprises a 50 .OMEGA. resistor and a thyristor diode as the threshold value circuit element for each line conductor. The protective circuit is constructed symmetrically for both line conductors. The thyristor diode dissipates excess voltage to ground. On the subscriber line side the resistor is inserted into the line conductor before the thyristor diode, and serves to limit the current in the event of an excess voltage.
Such a protective circuit must offer a certain protection against lightning, which is simulated as follows: an overvoltage source with a certain internal resistance and a certain impressed pulse- or sinusoidally-shaped overvoltage is applied to the subscriber line circuit for a certain period of time. This must not damage the subscriber line circuit.
Problems occur with the above protective circuit if the lightning protection requirement is increased, particularly with respect to the magnitude of the overvoltage. Because of the high requirements of resistance symmetry in subscriber line circuits, the two resistors are often realized in a thick-film hybrid technique and are balanced with a laser. This very much limits the voltage strength of these resistors which can only be increased by means of expensive technology.
The solution of this problem, as discussed in professional circles, is to insert a PTC in series with the resistor in each conductor line. However, such PTCs are relatively expensive and result in problems with the resistance symmetry.